1. Field of the Invention
The present invention relates to an operational amplifier, particularly to a battery-operated radio system.
2. Description of the Background Art
Amplifiers are required for a multitude of applications. Thus, amplifiers are used for filter circuits or for the amplification of measuring signals, for example, in sensor systems. A known amplifier circuit is, for example, the operational amplifier. Amplifiers can advantageously amplify a broad frequency range for a wide field of application. For some applications, it is also sufficient that the amplifier, as a selective amplifier, amplifies only a narrow frequency band within the range of an operating frequency.
An operational amplifier can have an input differential amplifier and an output stage. An operational amplifier is disclosed, for example, in “Analoge Schaltungen” [Analog Circuits], Seifart, 4th edition, Verlag Technik Berlin, 1994, pages 276 to 286. The input stage of an operational amplifier is, for example, a differential amplifier. A differential amplifier is disclosed, for example, in “Analoge Schaltungen” [Analog Circuits], Seifart, 4th edition, Verlag Technik Berlin, 1994, pages 107ff. The output voltage of the differential amplifier is proportional to the differential voltage between two input terminals. Common-mode voltages, which act at both inputs with the same amplitude and phase position, are not amplified by the ideal differential amplifier. The advantageous properties are conferred on the differential amplifier by its largely symmetric structure. The emitters of both input transistors can be connected to one another and to an input constant current source.
A low-power-2.4-GHz transmitter/receiver-CMOS IC with a differential amplifier is known from IEEE Journal of Solid-State Circuits, Vol. 38, No. 2, February 2003; pages 176ff. A 5.2-GHz, low-noise amplifier in 0.35 μm CMOS technology with a differential amplifier is known from IEEE Journal of Solid-State Circuits, Vol. 38, No. 4, April 2003.
A 3V CMOS Rail-to-Rail Input/Output Operational Amplifier for VLSI Cell Libraries is disclosed in IEEE International Solid-State Circuits Conference, 1994, Paper FA 14.1. A 1.2-V CMOS Operational Amplifier with Dynamically Biased Output Stage is disclosed in IEEE Journal of Solid-State Circuits, Vol. 35, No. 4, April 2000.
An electronic output stage for CMOS-LVDS levels (LVDS=low-voltage differential signaling) for use in analog and digital high-frequency circuits is known from patent No. WO 01/73943 A1, which corresponds to U.S. Pat. No. 6,781,449. The output stage has a first and a second transistor, which are connected with a first terminal to a current source and with a control terminal to input terminals. A third and fourth transistor are connected with a first terminal to a supply potential, with a second terminal to a second terminal of the first and second transistor, as well as to an output terminal, and with a control terminal to converted input signals. A differential amplifier, which is connected to the control terminal of the third and fourth transistors, is provided to convert the input signals. The differential amplifier is used to amplify and invert the input signals and to provide them with a voltage offset. It is also possible to set the desired output voltage at the output of the output stage via the high and low levels from the differential amplifier.